Due to increasingly stringent sulfur specifications in transportation fuels, removal of sulfur compounds from petroleum feed stocks and products has become very important among the refining processes. Environmental protection agencies worldwide have proposed severe regulations for limiting sulfur content in transportation fuels. In diesel, for example, this has already been reduced to 350-500 ppm in most of the countries and is being brought down to 50 ppm and further down to 10 ppm in some countries.
Desulfurization of diesel streams below 350 ppm involves generally the removal of residual sulfur compounds which are primarily aromatic heterocyclic compounds namely benzothiophene, dibenzothiophene and their alkyl substituted analogues or higher homologues. Amongst these, benzothiophene and dibenzothiophene are relatively easily removable sulfur compounds compared to their derivatives especially mono and di alkyl substituted. These sulfur compounds are refractory in nature and pose difficulties in their removal and is the limiting factor for producing ultra low sulfur diesel. Therefore, there is a need for catalysts having improved properties for effective desulfurization of these refractory sulfur compounds, so that sulfur level of the diesel fuel can be brought down to meet the strictest limit required by environmental regulations with minimum or no increase in catalyst volume and operating severity.
Deep desulfurization of diesel fuels requires the removal of the difficult to remove refractory sulfur species. Mochida et al (Catalyst Today, 29, 185, 1996) illustrates the importance of both the process and catalyst design for handling such refractory sulfur species. The approaches mentioned therein include selective oxidation of the refractory sulfur compounds or selective isomerization and/or cracking of the refractory compounds followed by hydrodesulfurization.
U.S. Pat. No. 5,958,224 discloses that after removal of easy sulfur using conventional hydrodesulfurization catalysts in the first stage, refractory sulfur can be removed by selective oxidation with transition metal oxide as an oxidizing agent. As an instant of this invention, this prior art uses a peroxo metal complexes in the second stage.
Another prior art disclosed in U.S. Pat. No. 5,897,768 for handling refractory sulfur using mixed catalyst bed consisting of conventional hydrotreating catalyst and a zeolite based selective isomerization and cracking catalyst. This is similar in approach with previous art except that after the removal of the non refractory sulfur using conventional hydrodesulfurization catalysts, the product stream containing the unconverted refractory sulfur is to be passed through a “ISOM” unit containing a USY zeolite catalyst where refractory sulfur compounds isomerize and disproportionate to easily removable sulfur compounds. Subsequently, this “ISOM” product is recycled to conventional hydrodesulfurization catalyst bed resulting to low sulfur product.
Various methods of preparation have been made in the art over the years to provide hydrotreating catalysts with improved activity and also for the removal of the most difficult to remove refractory sulfur species. The improvements of activity of hydrotreating catalysts are achieved either by improvement of the support characteristics or by improvement of active metal dispersions or both. This is achieved in the art by modifications of preparation procedures or by the use of additives.
U.S. Pat. Nos. 6,015,485, 6,200,927 and 6,239,054 disclose a method for modification of the support by way of generation of nanocrystalline phase on the alumina support. The method herein involves the modification of alumina carrier by use of chemical treatment, involving the use of chelating agent like EDTA and subsequent aging of the support for considerable period of time producing crystallite size <25 Å on the surface. This is responsible for higher activity catalyst.
Carriers have also been developed with controlled pore size distribution, surface area and surface acidity. This has been achieved in the art by the use of various additives and modification of preparation procedures. U.S. Pat. No. 3,969,273 discloses the use of 1-10% phosphate in calcined alumina support before the catalytically active metal is impregnated to achieve higher surface area and macro porosity. U.S. Pat. No. 3,969,273 discloses a method for preparation of phosphated alumina extrudates that can be used as support for hydrotreating catalyst preparation, the patent describes that the mulling of alumina powder with water soluble phosphate compound followed by extrusion generating supports with higher surface area and porosity.
U.S. Pat. No. 5,609,750 discloses a hydrotreating catalysts whose support constitutes of alumina, silica alumina and boron, the active metal are impregnated on to this modified support. The boron and silica-alumina is claimed to generate controlled acidity, which is responsible for some cracking activity of the catalyst. U.S. Pat. Nos. 5,151,172, 4,500,645, JP-V-2-39305 discloses the use of zeolites along with alumina support for generating acidity to the support. In all these patents the zeolites along with the alumina is extruded and then loaded with the active metal components.
Even though the above cited references show a continuous refinement and modification of methods to achieve a high active deep desulfurization catalyst from the point of view of support types, support structures, active metal components and their dispersion and loading, there is continuing need for better methods/procedures to achieve an optimum effect of all these parameters for preparation of higher active catalysts.
The present invention provides a high active hydrotreating catalyst by a process that results in well dispersed metal components in nanocrystalline range along with an acidic property of the support for effective removal of refractory sulfur compounds.